1. Field of the Invention
The present invention relates to a backlight unit (BLU), and particularly, to a backlight unit for a liquid crystal display device including cold cathode fluorescent lamps.
2. Discussion of the Related Art
Presently, display devices serve as very important visual information transfer mediums. Display devices must be light weight, thin and small to play a key role for future applications. Display devices can be categorized into different types. One type is the luminescent type that produces light by themselves, such as cathode ray tube (CRT) devices, electro-luminescent (EL) displays, light emitting diode (LED) devices, vacuum fluorescent display (VFD) devices, field emission display (FED) devices, and plasma display panel (PDP) devices. Another type is the non-luminescent type that cannot produce light by themselves, such as liquid crystal display (LCD) devices.
The LCD devices draw great attention as one of the next generation display devices along with PDP devices and EL displays. Images are displayed on LCD devices by making use of optical anisotropy of liquid crystals together with a backlight. As a result, a liquid crystal display can have a high level of visibility, low average power consumption and only releases a small amount of heat in comparison with an existing cathode ray tube (CRT) having the same size screen.
In LCD devices, the liquid crystals do not emit light, but receive, modulate and transmit light through a display panel. More specifically, data signals in accordance with image information are individually supplied to pixels arranged in a matrix configuration on the LCD panel such that light transmittances of the pixels are controlled to display the desired images. Accordingly, the LCD device requires a light source, such as a backlight unit, to irradiate light onto an LCD panel.
The LCD device has a liquid crystal display panel for providing an image that includes an array substrate, a color filter substrate, and liquid crystal injected between the two substrates. A backlight unit installed at the rear of the LCD panel emits light toward the entire front of the LCD panel. A plurality of case elements couple the LCD panel to the backlight unit. In the LCD panel, a pixel electrode on the array substrate and a common electrode on the color filter substrate are formed to apply an electric field across the liquid crystal layer. If a voltage of the data signal supplied to the pixel electrode is controlled such that the voltage is applied across the liquid crystal layer to the common electrode, the liquid crystals of the liquid crystal layer rotate by dielectric anisotropy along the electric field between the common electrode and the pixel electrode. Thus, light is transmitted or blocked by each pixel according to the rotation of the liquid crystal, such that characters or images are displayed.
The backlight unit 20 functions to provide planar light having uniform brightness from a fluorescent lamp 43 used as a light source. The thickness and power consumption of the LCD device is dependent upon the profile thickness and light efficiency of the backlight unit 20. In general, there are two types of backlight units. The first type of backlight is the direct type backlight in which a fluorescent lamp is positioned at the rear surface of the LCD panel to transmit light directly to the LCD panel. The second type of backlight is the edge type backlight in which a fluorescent lamp is positioned at one side or both sides of the LCD panel and light is reflected, diffused and concentrated by a light guide plate, a reflection sheet and other sheets to transmit light to the LCD panel.
The edge type backlight can be easily fabricated. Further, the edge type backlight typically has a profile thinner than the direct type backlight. However, light can be more readily distributed in a large LCD device with a direct type.
A cold cathode fluorescent lamp (CCFL) is commonly used as a light source in the backlight unit. The CCFL can easily be used in the edge type backlight but can not be used in the direct type backlight unit. This is because the CCFL is installed with a method in which soldering is performed between a lamp electrode and a lamp wire, and a silicon rubber covering is positioned to encompass the soldered portion. When the CCFL is used for the direct type backlight unit provided with several lamps, it takes a long time to perform the several individual installations of the lamps by means of soldering and silicon rubber coating. In other words, individual soldering of the CCFL is impractical as an integral type lamp holder is usually used for direct type backlight unit.
A related art CCFL and method of driving will now be described in detail with reference to FIGS. 1A, 1B, 2A, 2B.
FIGS. 1A and 1B are schematic views of a related art CCFL and a driving method thereof. As shown in FIGS. 1A and 1B, a high voltage is applied across the CCFL 40A via the internal electrodes 41 arranged at both ends of a CCFL 40A, and the voltage is raised up to a starting voltage which enables current to flow through the CCFL 40A. When the voltage exceeds the starting voltage, light emission occurs in the CCFL 40A. Then, a continuous emission of light is maintained by applying an alternating current (AC) to the CCFL 40A.
In the edge type backlight, the operation is only for one CCFL 40A. The operation would have to be individually performed as frequently as the number of CCFLs in the direct type backlight unit. Accordingly, a backlight unit employing the CCFL is usually only fabricated as an edge type. External electrode florescent lamps (EFFLs) are usually used for the direct type backlight unit.
FIGS. 2A and 2B are schematic views of a related art external electrode florescent lamp and a driving method thereof. Unlike the CCFL, an EEFL 40B has no internal electrodes or electrodes that protrude inwardly at both ends of a glass tube. A conductive material is applied on both ends of the EEFL 40B. The EEFL 40Bb emits light by a driving method in which ions polarized by the electric field change created by external electrodes 42 gather at both ends according to their polarities and then are recombined at the point of the zero crossing due to a high-voltage AC.
Because the electrodes of an EEFL are external, an equivalent circuit of set of EEFLs is a set of parallel capacitors. Accordingly, several EEFLS 40B may be driven in parallel. Once a parallel set of EEFLs is provided with a voltage inverter of sufficient capacity, the EEFLs may emit light with a simpler structure and a simpler inverter in comparison to the structure and inverter that would be necessary to drive a set of CCFLs. However, the EEFL obtains a sufficiently high brightness level by a high-frequency driving of at least a few MHz. The high-frequency driving of the EEFL causes problems, such as electro-magnetic interference due to the high frequency, low efficiency as well as disadvantages associated with a high frequency power supplying unit.